1. Field
Embodiments relate to a process for forming a nanoantenna array, a nanoantenna array chip and a structure for lithography, which is used for forming a nanoantenna array.
2. Description of the Related Art
A metallic nanostructure shows a localized surface plasmon resonance phenomenon in a specific wavelength region due to a dielectric confinement effect of free electrons. At a resonance wavelength, a local electric field near a metallic nanostructure is strongly enhanced, and a characteristic optical absorption or scattering behavior occurs in proportion to the intensity of the local electric field.
Since the characteristics of localized surface plasmon resonance sensitively depend on the change of surroundings, sensor application using the same has been extensively studied. For example, it is typical that the effect of local electric field enhancement is utilized in plasmonic substrates for a surface enhanced spectroscopy which amplify a Raman signal or a fluorescence signal arising from the molecules present near the metallic nanostructures, and a sensitive response of resonance wavelength to the change in an effective refractive index of surrounding media is used for a localized surface plasmon resonance sensor to detect an analyte near the metallic nanostructures.
Recently, an infrared plasmonic nanoantenna technology is in the spotlight for its potential to amplify a spectroscopic molecular absorption signal by matching the vibrational modes inherent to a molecule to be analyzed with a plasmonic mode of a metallic nanostructure or give a way of using a plasmon energy transition into the molecule, thereby enabling the highly sensitive and selective detection of analytes. Most molecules have inherent vibrational modes in the infrared wavelength region, but the infrared wavelength is much greater than the size of molecules. Thus, the molecular absorption coefficient in the infrared region is generally low. However, if the plasmonic nanoantennas are used, the size mismatch may be overcome by means of a local electric field concentrated on the surface of nanostructures, and thus infrared absorption signals of adjacent molecules may be amplified.
In this case, since most molecules have their inherent vibrational modes in an infrared wavelength region, specifically in a mid-infrared region between 2 μm and 20 μm called a molecular fingerprint region, as described above, the resonance wavelength of nanoantenna needs to be tailored in the infrared region to match the plasmonic modes of metallic nanostructures with the vibrational modes of a molecule to be analyzed. In order to facilitate the shift of resonance wavelength of nanoantennas to the infrared region, the nanostructures should have an anisotropic shape, and particularly in the case of a nanorod structure known for its excellent wavelength tunability, the aspect ratio which is a ratio of long axis to short axis needs to be increased.
Plasmonic infrared nanoantennas reported until now have been mostly fabricated using E-beam lithography. The E-beam lithography has an advantage in that a nanostructure of a desired shape may be precisely fabricated, but it requires expensive processing equipment and is not suitable for fabricating a large-area array chip due to its low throughput. Meanwhile, nanoimprint lithography may resolve the issue of low-throughput of the traditional E-beam lithography by repeatedly replicating nanopatterns with a stamper. Even in this case, however, fabrication of the stamper still relies on the E-beam lithography that is expensive and time-consuming, and once the stamper pattern is fixed, versatile modification in the pattern is impossible. Also, the pressure and temperature must be kept uniform throughout the imprinting process. Therefore, this method still has limitations in terms of overall costs and large-area processing.
As an alternative, U.S. Patent Publication No. 2013/0148194 discloses a method for forming a plasmonic nanoantenna using a nanostencil lithography. In this method, an array of nanostructures can be simply transferred onto a substrate just by depositing a target material over a free-standing stencil mask precisely perforated to have nano-pattern pores, similar to a traditional stencil printing method. This process itself is simpler than the nanoimprint method, but it requires both an expensive E-beam lithography and a micro-electromechanical system (MEMS) process to make the nanostencil mask. In addition, it is impossible to give a modification in structure, other than a given pattern, and not suitable for a large-area process due to the free standing structure of nanostencil.
The resonance wavelength of a nanoantenna may be easily tailored to match the inherent vibrational modes of molecules constituting an analyte by controlling a shape of the nanoantenna. Therefore, there exists a need for a low cost process for a large area fabrication of nanoantenna arrays which enables a versatile control of the resonance wavelength on request.
U.S. Unexamined Patent Publication No. 2012/0258289 discloses a method for fabricating an array of anisotropic nanostructures based on a low cost self-assembly process. This method makes it possible to fabricate anisotropic nanostructures by modifying an existing patterning process which utilizes a photonic nanojet, generated when a transparent dielectric micro-bead is illuminated by light and having a sub-diffraction-limit spot size, to sensitize a photoresist located below the dielectric micro-bead. In detail, by adding a linear diffuser which scatters light only in one direction, the focused light forms an anisotropic spot pattern on the photoresist layer and finally a nanorod type structure is implemented. This document discloses a method for adjusting a divergence angle of the diffuser to increase an aspect ratio of the nanorod but does not disclose a method for accomplishing the aspect ratio high enough to shift the resonance wavelength to cover an infrared region of 2 μm to 20 μm for the application into an infrared nanoantenna and various structural engineering techniques for improving functionality of the nanoantenna.